What causes 'illegalstateexception in java'?

Example:

List list = new ArrayList<>();
Iterator it = list.iterator();
list.add(4);
int value = it.next();

Solution:

This error typically arises when you modify a collection while iterating over it. Use a different approach, like using `CopyOnWriteArrayList` or modifying after iteration.

List list = new ArrayList<>();
list.add(4);
Iterator it = list.iterator();
int value = it.next();